The identity marking of production goods is becoming increasingly important across almost all sectors of industry. For example, it is frequently necessary to apply production dates, expiry dates, bar codes, company logos, serial numbers, etc., to plastics or polymer films. At the present time, these marks are made predominantly using conventional techniques such as printing, embossing, stamping and labelling. Growing importance is being acquired, however, by contactless, high-speed and flexible marking using lasers, especially in the case of plastics. This technique makes it possible to apply graphic inscriptions, for example bar codes, at high speed even to a non-planar surface. Since the inscription is within the plastics article itself, it is durably abrasion-resistant.
Many plastics, such as polyolefins and polystyrenes, for example, have hitherto proved to be very difficult or even impossible to mark by means of the laser. A CO.sub.2 laser which emits light in the infrared region at 10.6 m produces only a faint, barely legible mark on polyolefins and polystyrenes even at very high output levels. In the case of the elastomers polyurethane and polyether esters, there is no interaction with Nd-YAG lasers whereas with CO.sub.2 lasers engraving occurs. The plastic must not completely reflect or transmit the laser light, since if it did so there would be no interaction. However, there must also not be strong absorption, since in that case the plastic would evaporate to leave only an engraving. The absorption of the laser beams, and hence the interaction with the material, is dependent on the chemical structure of the plastic and on the laser wavelength employed. In many cases it is necessary to add appropriate additives, such as absorbers, in order to render plastics laser-inscribable.
For the laser identity marking of plastics, use is increasingly being made, besides CO.sub.2 lasers, of Nd:YAG lasers. The YAG lasers commonly used emit a pulsed energy beam having a characteristic wavelength of 1064 nm or 532 nm. The absorber material must exhibit pronounced absorption within this specific NIR range in order to show a sufficient reaction in the rapid inscription processes.
The laser marking of plastics with pearl luster pigments was first described in Speciality Chemicals, Pearl Lustre Pigments--Characteristics and Functional Effects--May 1982.
The use of organic additives as dopants in laser marking is known, for example, from WO 95/30716, DE 195 360 47 A1, US 5,576,377 and DE 197 26 136 A1.
The prior art dopants all, however, possess the disadvantage that they persistently color the plastic that is to be inscribed, as a consequence of which the laser inscription, which is normally a dark script on a light background, lacks a sufficient contrast. Moreover, they have to be added in comparatively high concentrations, and in many cases are toxicologically objectionable.